2L 4Cyl. vs. 2L 8Cyl What's more fuel efficient? - Page 3 - Fuel Economy, Hypermiling, EcoModding News and Forum

serialk11r · 04-27-2012, 07:19 PM

Quote:

Originally Posted by 3-Wheeler

What Mr Miller pointed out earlier, multi engines are preferred for more power output, since they allow higher valve area per cylinder volume, and higher rpm's, BUT at a lower operating efficiency. Back in the late 60's Honda was famous for it's family of multi-cylinder race engines, including a 250cc 6-cylinder.

Is it the valve area per cylinder volume or is it the balance aspect? You can increase valve area by shortening stroke and increasing bore, but then I guess there are combustion issues when the bore gets too big or something.

I believe 6 cylinders are used in luxury cars because they are smoother, and I've heard that the reason for V10 and V12 rather than a fat V8 is because of better balance or something, not sure. They sure do make a great sound

tortoise · 04-27-2012, 07:34 PM

Quote:

Originally Posted by Gealii

im going with the I4 cuz that straight motors tend to put out alot of torque which would allow the user to hold a speed easier but accelerate slower

Do you think an inline 3 might work?

drmiller100 · 04-27-2012, 08:30 PM

Quote:

Originally Posted by tortoise

Do you think an inline 3 might work?

absolutely. an inline 3 can actually be more "ballanced" then an inline 4. There are harmonic issues with 4 cylinders to where 2.5 liters is about the max you ever see for an line 4 cylinder.

user removed · 04-27-2012, 09:32 PM

The 4 cylinder engine has the advantage of sequential induction pulses. There is always another induction pulse for every 180 degrees of crankshaft revolution. This reduces the pressure waves in the intake manifold and makes intake flow much smoother. More than 4 cylinders will enhance the effect but it takes at least 4 to make the induction flow fairly consistent.
Do you want power or torque? A 600 CC motorcycle engine at 15,000 RPM makes ungodly power. A 2.4 liter long stroke engine makes gobs of torque. The latter will give you more power for the same amount of fuel.
Fewer cylinders (but more than 4) will give you less surface area of the cylinder head and cylinder walls for heat to dissipate. The 3 foot bore and 9 foot stroke of the most efficient engines on the planet give you and idea of how to build them more efficient but they run at 100 RPM. The also have a scotch yoke to eliminate the side loads on the piston.
regards
Mech

3-Wheeler · 04-28-2012, 12:16 AM

Quote:

Originally Posted by smokey442

BMW published a paper in this subject including optimum bore and stroke. The v8 wins out for reasons of lower heat rejection rate and accessory losses (oil pump and waterpump)

Consider the quote "The V8 wins out for lower heat rejection rate..." ...

This goes contrary to typical thermodynamics text books.

I also have direct experience with $15,000 pressure decay testers that say that heat rejection would go up (be quicker) with more cylinders (larger surface area to the cooling air).

It is well proven that for higher fuel economy, less cylinders equals more. You want to maintain the heat in the cylinder for moving the piston down, not heat the air around the outside of the cylinder, and waste it to the air flow.

Jim.

3-Wheeler · 04-28-2012, 12:30 AM

Quote:

Originally Posted by big shafe

Do you have any or can point me to any data that supports this? I would think temperature gradients would be hard to quantify. I also think it would be highly dependent on fuel/air mixture distribution and fluid flow characteristics.

I could possibly see that for similar compression ratio's but not sure about large compression ratio differences.

I remember as a kid, we had a toy that contained about 6 plastic eggs, the smallest one inside the next bigger one and so on.

Now picture the six eggs as being "layers" of freshly heated air inside an engine cylinder.

The smallest egg is the hottest since that egg is insulated by the thermal ability of each larger egg. Each egg is actually a "thermal blanket" insulating the cooler cylinder wall from the "hot egg in the center".

Since we know that the air itself is a natural heat or thermal insulator, the core heat inside the cylinder is maintained longer by more layers of "eggs" to keep the core heat from cooling much.

Now let's correlate our six eggs with a relatively small displacement cylinder. How much longer will the same gas stay hot if we make the cylinder larger by adding more layers of "eggs" to the mix?

What if we now have, in addition to the six layer egg, introduce a nine layer egg as well. Remember, if a layer of egg represents more layers on insulating "air", then the egg with more layers will certainly maintain it's trapped heat longer after the spark plug fires the fuel-air vapor mix. The six layer egg represents a small cylinder size and the nine layer egg equals a larger cylinder.

Our egg experiment is exactly one of the reasons why an engine with larger cylinders maintains it's core heat longer. More egg layers.

Jim.

Duffman · 04-28-2012, 01:39 AM

Less cylinders for FE always wins, not only the lower surface area for the volume but also less friction from cams and crank bearings.

HP is is on the other end of the spectrum, HP is limited by airflow which is controlled by bore size (in relation to stroke) in a Naturally aspirated engine. More cylinders are also beneficial in this regard.